The present invention relates to a radio pager having a frequency correcting function.
One of conventional schemes for demodulating an FM (Frequency Modulation) multilevel digital signal uses the characteristic of a discriminator connected in parallel with a damping resistor. This scheme transforms the FM signal to a voltage and then demodulates it on the basis of the characteristic of the discriminator. A multilevel signal representative of the above voltage is subjected to decision using preselected levels (voltages) at the subsequent stage. However, a problem with this discriminator scheme is that the phase characteristic of the discriminator is susceptible to changes in temperature and power source voltage. As a result, an IF (Intermediate Frequency)-to-DC voltage characteristic (f-V characteristic or S curve characteristic) derived from the phase characteristic of the discriminator is not constant. Another problem is that when a signal is offset at a transmitter side or when a reference oscillation section at a receiver side is influenced by, e.g., environment or inaccurate adjustment, the IF frequency varies at the output of a mixer and obstructs accurate multilevel decision to follow.
Japanese Patent Laid-Open Publication No. 58-14618 proposes a method of controlling a voltage controlled oscillator such that the mean frequency of the IF signal produced by frequency conversion coincides with the IF center frequency. For this purpose, a difference between the mean frequency of the IF signal and the IF center frequency is negatively fed back to the frequency control signal input terminal of the voltage controlled oscillator.
To correct the phase characteristic of the discriminator susceptible to temperature, the damping resistor connected in parallel with the discriminator may be replaced with a thermistor having a B constant, as also proposed in the past. The correction, however, cannot be done unless the B constant characteristic of the thermistor and the phase characteristic of the discriminator are matched.
When the frequency-to-phase (f-.theta.) characteristic varies due to the scatter of the discriminator ascribable to temperature or power source voltage, the f-V characteristic of a modulation circuit also varies. It has been customary with a receiver to mix the output of a local oscillator with a high frequency received signal in order to produce an IF signal. At this instant, the local oscillator is often adjusted by, e.g., a manually variable capacity implemented by a quartz oscillation circuit or a voltage controlled oscillator.
However, assume that the f-.theta. characteristic of the discriminator varies and causes the output voltage at the center frequency of the f-V characteristic converted by the discriminator to vary, that the transmission frequency varies due to offset transmission from a transmitter side, and/or that the local oscillator at a receiver side varies due to environment or inaccurate adjustment. Then, the IF frequency signal output from the mixer is shifted from the expected center frequency or the f-V characteristic of the discriminator itself varies. This prevents an accurate voltage amplitude and accurate multilevel decision from being guaranteed.